To continue to make progress in Pediatric Oncology-Hematology (POH), we need to perpetuate the pool of talented and trained POH physician-scientists and PhD scientists who will make POH-relevant discoveries and provide investigative leadership. This has become ever more challenging in the field of POH, as there continues to be a shortage of young investigators with extensive laboratory research expertise. Fortunately, we are able to attract candidates willing to spend the time in the laboratory until the tutelage of outstanding investigators who are able to inculcate them with the culture of research that predominates at JHMI and the NCI, despite the current challenging funding climate. This grant and the philanthropic funds that we raise enables us to guarantee them at least 3 years in the laboratory, with many staying on at the Instructor level for additional time until they are fully ready to establish their own lab. In the current funding period (2012-2017), this grant has already supported superb laboratory research training for an outstanding group of 23 physician-scientists and 5 PhD scientists who have demonstrated a high degree of productivity and commitment to careers in cancer research. In addition, these trainees have strengthened the laboratory research base in POH in the Johns Hopkins Medical Institutions (JHMI) and the POB of the NCI. This application seeks support for 10 interdisciplinary postdoctoral research fellowship positions to fund the research training that is the major ingredient of the combined Johns Hopkins University/National Cancer Institute (NCI) Subspecialty Fellowship Training Program in POH (Joint Fellowship). We previously had 10 funded positions which were reduced to 8 because of NCI budgetary considerations despite a perfect score during the last renewal. This forced us to reduce the number of starting trainees in the Joint Fellowship training program from 7 to 6. Returning to 10 training slots will allow our program to provide sufficient duration of mentored research for the physician-scientists that will become leaders in the field. Two groups of postdoctoral fellows will be supported by the grant proposed herein. The major group of trainees will be composed of physician-scientists, since the laboratory research training of academic POH physician- scientists is the primary objective of this training program. These highly selected physician-scientists will have completed the clinical year of their fellowships in the Joint Fellowship. The second (smaller) group of trainees will be composed of PhD postdoctoral scientists working at JHMI on projects highly relevant to the biology of childhood cancer and blood diseases. These strongly qualified trainees will be provided intensive 3-year research experiences that will prepare them for careers in cancer research. Trainees work on projects relevant to the biology of childhood cancer and blood diseases, in the laboratories of leading scientists. The trainees and their mentors participate in an interactive, dynamic research environment bridging the JHMI and the NIH, with an emphasis on translational research.